I. Field of the Invention
This invention relates to a process for preparing silicon carbide platelets, to the platelets thus formed and to the use of the platelets for particular applications.
Silicon carbide "platelets" are single crystals of SiC having two dimensions appreciably greater than the third. Generally, platelets have an aspect ratio (i.e. length or width to thickness) greater than 3 and preferably greater than 5.
II. Description of the Prior Art
Silicon carbide in the form of whiskers, i.e. single crystals grown primarily in one dimension, can be used as reinforcing materials for matrices made of various materials, particularly ceramics and metals. However, silicon carbide whiskers are suspected of being health hazards because, like asbestos fibres, they easily become airborne and can be ingested by humans or animals. There is therefore a need for an alternative to silicon carbide whiskers that offers similar reinforcing effects without the associated risks.
Silicon carbide platelets have come under consideration as alternatives to whiskers for matrix reinforcement, but the platelets produced by the conventional method of reacting silica and carbon at high temperature are generally highly agglomerated and cannot easily be separated. Agglomerated platelets are not very useful as reinforcing materials because they remain in clumps in the matrix and are difficult to disperse. Moreover, the resulting platelets tend to be thicker than desired for an optimum reinforcing effect.
For silicon carbide platelets to be useful in applications such as reinforcement of ceramic or metal matrix composites, we have found that they must posses specific attributes, namely:
(a) they must be substantially completely unagglomerated; PA0 (b) the size should preferably be in the range of 5-50.mu., and preferably 10-20.mu.; PA0 (c) the aspect ratio (width/thickness) should desirably be greater than 5, at a diameter of 10-20 microns.
For the platelets to possess these attributes, the assynthesized product must be substantially non-agglomerated and the size of the platelets should not exceed 50 microns and preferably 30 microns. Even though it is possible to reduce the size of larger platelets by attrition to the required dimensions, this is undesirable because it results in an undesirable reduction in the aspect ratio.
U.S. Pat. No. 4,756,895 to Boecker et. al., which issued on July 12, 1988, discloses a process for producing crystalline silicon carbide platelets. The process involves heating a porous silicon carbide precursor composition comprising silicon and carbon in intimate contact to a temperature of from 2100.degree. C. to 2500.degree. C. in a non-reactive atmosphere in the presence of a hexagonal crystal growth control additive. The silicon carbide precursor may be a blend of carbon with silica, the silica being in the form of particles, e.g. fume silica having a particle size of 0.005 to 100 microns, or a sol. The carbon may be carbon black or lamp black, i.e. colloidal size carbon having a particle size of about 0.005 to 2 microns. The hexagonal crystal growth control additive is a Group IIIA metal such as boron or aluminum, or mixtures thereof. Although the process of U.S. Pat. No. 4,756,895 appears to be a useful way of producing silicon carbide platelets, we have found that it does not yield unagglomerated silicon carbide platelets of the optimum size and aspect ratio greater than 5 required for good reinforcement effects and therefore believe that there is a need for an improved process of this type.